Apparatus which uses nuclear magnetic resonance (NMR) phenomena to measure and produce images of the distribution of physical properties in body tissue is well known and is described, for example, in the article Proton NMR Tomography, P. R. Locher, Philips Technical Review, Volume 41, 1983/84, No. 3 at page 73. This apparatus usually includes a magnet system for generating a large, homogeneous magnetic field in conjunction with a plurality of smaller gradient magnetic fields, means for generating a homogeneous radio frequency alternating magnetic field which excites nuclear magnetic resonance phenomena within the body, and means for detecting magnetic resonance signals which are generated within the tissues being examined. This apparatus allows simultaneous measurement of the properties at many points within a region of the body.
In such apparatus, it has become common to use a first large coil to generate the exciting radio frequency magnetic field and a second, smaller coil, often called a "surface coil," to detect resonance signals in a localized volume of the body which is undergoing examination. Apparatus of this type is described in Computer Tomography, 1, 1981, pp. 2-10 and at chapter 8, page 164 of the book Nuclear Magnetic Resonance and its Applications to Living Systems, D. G. Gadian, Clarendon Press Oxford. (1982) which describes the advantages of using surface coils for bloodstream measurements. Surface coils are said to achieve high detection signals-to-noise ratios and to make accurate measurements in relatively small regions near the coil, since it is possible to adapt the shape and size of the coil to the shape and size of the object being measured. In such systems, the radio frequency electromagnetic coil is usually a larger coil, often called a "body coil", in order to achieve sufficient spatial homogeneity of the magnetic field.
Direct electromagnetic coupling between a surface coil and the body coil can degrade performance of a magnetic resonance imaging system, particularly when the body coil and surface coil are not disposed in perpendicular planes. U.S. patent application Ser. No. 677,939 (filed Dec. 4, 1984 by E. Boskamp et al) describes apparatus which electrically decouples a surface coil from a body coil. In one embodiment of this apparatus, an electrically-variable capacitor is used to detune the surface coil by raising its resonant frequency above the nominal signal frequency whenever the body coil is activated. This system is also described in the article Application of Surface Coils in MR Imaging by E. Boskamp in Medicamundi, Vol. 29, No. 2, 1984. The above-mentioned publications are incorporated herein, by reference, as background material. PG,4
Prior art surface coils have generally been constructed from relatively rigid copper wire. Single-turn surface coils were preferred at the frequencies which are used in present-day magnetic resonance imaging systems. It is known that surface coils should be constructed from materials which are free of paramagnetic impurities. Copper coils have, therefore, been used in preference to silver coils, even though silver has better electrical conductivity properties than does copper.